I think this prompt means to be magnanimous, designed to allow a modern audience to pick one ol' time movie they like, something Turner Classic Movies would show, presuming the meme writer watches mostly current stuff. But tonight, I'm in an irascible and impatient mood, and I find it a bloody annoying prompt. "Classic" is such a pesky word. It's not helped by the fact that many of the films that are considered famous classics I don't like (movies like Casablanca, The Third Man, Citizen Kane, Dr. Zhivago, Touch of Evil, Shane, High Noon, The Maltese Falcon, Notorious, 2001: A Space Odyssey... yeah, yeah, yeah, it's true. I don't like any of those movies. Shocked? Well, I warned you I was feeling cranky!)
So, to answer the darned question:
Before 1970: Paths of Glory (1957) - Just about the only Stanley Kubrick movie I not only like, but am in awe of it. Powerful, powerful movie with a great cast that just hits me in the gut every time and gets better every time I see it.
Post 1970: Star Wars (1977) - The movie love of my life, that changed my life. There's before-SW and after-SW, but I often think you really have to be about my age to get what that truly means. Seeing those previews on the little tv and going what the heck is that furry roaring creature? and then waiting in huge lines forever and experiencing that film for the first time in the theater, no spoilers, no clue what was coming or even what it was about, and when that Imperial cruiser goes and goes, and then zowie! space battle! and the good guys lose, and then Darth Vader's entrance... holy smoke, there was no experience quite like that one. Or waiting in line during the weeks that followed with two hundred other mom's and their kids, waiting for JC Penney's to open in the morning so you could get in and buy a t-shirt and the newest Star Wars figures. Or having your mom drive you to the department store (!) to meet Darth Vader and get his autograph. While you return and see the movie again and again in the theater because your family just can't get enough, and you know you're never going to see it again except on tv where things are tiny and panned and scanned and you'll miss what's on the sides so you'd better keep going until you have every frame memorized, every sound effect imprinted, all the dialogue memorized, every note of music... (And then, lo and behold! VCRs come out, and then DVDs, and you can see the movie again, and wide-screen to boot, and then Lucas goes and changes the movie you loved with his so-called special editions, "special" my Aunt Jenny's fruitcake, and that's a whole other story, but it doesn't detract from the original experience.)
And how was that for a massive slew of run-on, stream-of-conscious sentences? LOL!